When a magneto-optical recording method is adopted, a recording medium which includes a substrate having formed thereon a magnetic thin film with perpendicular magnetization made of a magnetic substance is employed, and recording and reproducing operations on and from the recording medium are performed in the following manner.
When a recording operation is to be carried out, first, the direction of the magnetization in the recording medium is arranged in one direction (upward or downward) by applying thereto a strong external magnetic field, in order to initialize the recording medium. Next, a laser beam is projected onto a recording area of the recording medium so as to raise a temperature thereof above the vicinity of its Curie temperature or above the vicinity of its compensation temperature. As a result, coercive force (Hc) at the portion becomes zero or substantially zero. In this state, an external magnetic field (bias magnetic field) having an opposite direction to an initializing magnetic field is applied, thereby reversing the magnetization direction. After the projection of the laser beam is stopped, the temperature of the recording medium drops to a room temperature, and thus the reversed magnetization direction is fixed, thereby recording information thermomagnetically.
When a reproducing operation is to be carried out, a linearly polarized laser beam is projected onto the recording medium, and the recorded information is optically read out utilizing an effect that the polarization plane of the reflected light or the transmitted light rotates differently according to the magnetization direction (magnetic Kerr effect or magnetic Faraday effect).
The magneto-optical recording medium designed for the magneto-optical recording method has been viewed with interest as a rewritable high density and large capacity memory device. In order to rewrite on the magneto-optical recording medium, either one of the following methods is required:
(a) initializing through any method; PA1 (b) devising an external magnetic field (bias magnetic field) generation device so as to enable the overwriting operation (rewrite without requiring an erasing operation); and PA1 (c) devising the recording medium so as to enable the overwriting operation.
However, when the method (a) is adopted, either an initialization device is required, or two magnetic heads are required which increase a manufacturing cost. If an rewriting operation is carried out using only one magnetic head, the problem is presented in that a long time is required because a recording operation can be carried out only after carrying out an erasing operation. On the other hand, when the method (b) is adopted, the magnetic head may be crushed as in the case of the magnetic recording.
Thus, the method (c) of devising the recording medium is the most effective method. According to this method, by employing a double-layered exchange coupled film for a recording layer, the overwriting operation is enabled, for example, as disclosed in Jap. Jour. Appl. Phys., Vol. 28 (1989) Suppl. 28--3, pp. 367-370.
The processes for the overwriting operation will be briefly described below. As shown in FIG. 6, in the magneto-optical recording medium composed of a first magnetic layer 9 and a second magnetic layer 10, an initializing magnetic field H.sub.init is applied thereto so as to arrange the magnetization in the second magnetic layer 10 in one direction (downward in the figure) in order to initialize the recording medium. An initialization may be carried out whenever an operation is to be carried out or only when a recording operation is to be carried out. In this state, since a coercive force H.sub.1 of the first magnetic layer 9 is greater than the initializing magnetic field H.sub.init, the direction of the magnetization in the first magnetic layer 9 is not reversed as shown in FIG. 7.
A recording operation is performed by projecting a laser beam which is to be switched between a High level I and a Low level II, while applying a recording magnetic field H.sub.w.
Here, the High level I and the Low level II are respectively set as follows: when a laser beam of the High level I is projected, both the temperatures of the first magnetic layer 9 and the second magnetic layer 10 are raised to the temperature T.sub.H which is in the vicinity of or above the Curie temperatures T.sub.1 and T.sub.2 ; on the other hand, when a laser beam of the Low level II is projected, only the temperature of the first magnetic layer 9 is raised to the temperature T.sub.L which is in the vicinity of or above its Curie temperature T.sub.1.
As shown in FIG. 6, when a laser beam of the High level I is projected, the direction of the magnetization in the second magnetic layer 10 is reversed upward by applying thereto the recording magnetic field H.sub.w. Then, the direction of the magnetization in the first magnetic layer 9 is arranged in the direction of the magnetization in the second magnetic layer 10 by the exchange force exerted on an interface in the process of cooling off. As a result, the direction of the magnetization in the first magnetic layer 9 is arranged upward.
On the other hand, when a laser beam of the Low level II is projected, the direction of the magnetization in the second magnetic layer 10 is not reversed by the recording magnetic field H.sub.w. As in the case of projecting the laser beam of the High level I, the direction of the magnetization in the first magnetic layer 9 is arranged in the direction of the magnetization in the second magnetic layer 10 in the process of cooling off. As a result, the direction of the magnetization in the first magnetic layer 9 is arranged downward as shown in FIG. 6.
Additionally, the recording magnetic field H.sub.w is set significantly smaller than the initializing magnetic field H.sub.init. The intensity of the laser beam used in reproducing is set significantly lower than the lower level II used in recording.
However, when the above method is adopted, an extremely large initializing magnetic field H.sub.init is required because interfacial coupling force exerted between the first magnetic layer and the second magnetic layer 10 is large. If a combination of the first magnetic layer 9 and the second magnetic layer 10 which can make the required initializing magnetic field H.sub.init smaller is employed, an overwriting may not be performed.
In order to counteract the above problems, a recording medium having a triplilayer structure wherein an intermediate layer is provided between the first magnetic layer 9 and the second magnetic layer 10 has been proposed in order to make the required initializing magnetic field H.sub.init smaller.
For example, the Japanese Laid Open Patent Publication No. 239637/1988 (Tokukaishou 63-239637) discloses an intermediate layer made of a material which has an in-plane magnetization at room temperature. However, when adopting the above intermediate layer, the problem is presented in that the magnetization may not be copied from the second magnetic layer 10 to the first magnetic layer 9 desirably especially at high temperature.
The Japanese Laid Open Patent Publication No. 24801/1990 (Tokukaihei 2-24801) discloses another intermediate layer made of a material which has an in-plane magnetic magnetization at room temperature. Since the first magnetic layer 9 is rare-earth metal rich at room temperature, problems are presented, for example, in that the direction of H.sub.init is different from the direction of H.sub.w and that initialization may not be carried out desirably.
The Japanese Examined Patent Publication No. 22303/1993 (Tokukokuhei 5-22303) discloses still a another intermediate layer which is a magnetic layer having properties that it has an in-plane magnetization at room temperature, and a transition occurs therein from the in-plane magnetization to a perpendicular magnetization as temperature rises. However, when a recording medium including the above intermediate layer is used, although a recording bit can be improved in terms of its stability, the problem is presented in that a desirable reproducing signal quality may not be achieved.